1. Field of the Invention.
This invention relates to a process of forming a porous refractory mass and to a composition of matter for use in such a process.
Such a process is useful for the formation or repair of a thermally insulating lining or cladding on a surface such as the surface of a refractory wall of a furnace or other structure which will be exposed to high temperatures in use. Examples of such structures are glass-making furnaces, cracking furnaces as used in the oil industry, coke ovens and refractory equipment employed in metallurgy.
2. Description of the Related Art.
To form a thermally insulating refractory mass or lining on a surface such as, for example, a refractory wall, the common practice is to face that surface with porous, and therefore insulating, refractory material, for example in the form of bricks or small slabs. This operation is carried out with cold bricks and it implies the possibility of access by the brick-layer to the surface on which the mass is to be formed. An operation of this type cannot therefore be carried out in a hot location such as, for example, on a wall of a furnace at operating temperature. It will be appreciated that cooling of a furnace or other structure from its operating temperature to enable such re-bricking to take place, and subsequent reheating would subject the furnace to such thermal stresses that significant further damage could be caused and the furnace might well be in worse state after re-bricking than before. Such cooling and reheating would also add significantly to the time needed for carrying out the re-bricking, and cool re-bricking is accordingly an entirely unsatisfactory process, unless the furnace is, in effect, being rebuilt.
While it would in theory be possible to use a hot re-bricking technique in order to cut down the repair time, this would also give rise to problems which are in practice insurmountable. Remote handling equipment would be required for positioning the bricks or slabs and for cementing them in position. There is no such equipment in existence which is capable of working at many locations within large refractory structures. Even at relatively accessible locations hot re-bricking would be unsatisfactory because the refractory cement would not give a satisfactory bond between the new bricks themselves, or between the new bricks and the existing hot refractory structure, even if the new bricks were to have been preheated.
Of course there are known processes for effecting the hot repair of refractory structures. Probably the most commercially successful of such processes is one which has become known as "ceramic welding". Examples of such ceramic welding processes are described in Glaverbel's British Patents Nos GB 1,330,894 and GB 2,170,191. In the ceramic welding process, a refractory mass is formed on a surface by projecting against that surface, in the presence of oxygen, a ceramic welding powder which comprises a mixture of refractory particles and fuel particles: the fuel particles are of such a composition and size that they react exothermically with oxygen, forming a refractory oxide and releasing the heat needed to melt, at least superficially, the projected refractory particles so that the refractory particles and combustion product(s) cohere into a refractory mass. Aluminium and silicon are examples of suitable fuels. It is known that silicon ought, strictly speaking, to be categorized as a semimetal, but since silicon behaves like some metals (it is capable of undergoing a highly exothermic oxidation to form a refractory oxide), for reasons of convenience these fuel elements are often referred to as metallic. It is generally recommended to project the ceramic welding powder mixture in the presence of a high concentration of oxygen, for example by using oxygen of commercial grade as carrier gas. A coherent refractory mass is thus formed, which can adhere to the surface against which the particles are projected. The exothermic reaction zone of the ceramic welding process can reach very high temperatures which make it possible to burn through any slag which may be present on the target surface and to soften or to melt that surface. A good joint is thus produced between the surface which is treated and the newly formed refractory mass.
This ceramic welding process can be used to form a refractory element, for example a block of special shape. However, it is most commonly employed for forming linings or performing repairs on blocks or walls. It is particularly useful for repairing or strengthening existing refractory structures by the in situ formation of a high quality compact and coherent refractory weld mass. It is quite commonplace to carry out this operation when the base refractory is hot and, in some cases, it is even possible to perform this repair or this strengthening without the need to stop the operation of the device. Indeed, in general, the hotter the target refractory surface, the more efficient is the ceramic welding process and the better is the bond between the weld mass formed and the pre-existing refractory structure.
The ceramic welding process owes much of its success to the fact that a major proportion of the combustion of the fuel particles takes place against the target surface. Thus the maximum amount of heat is available actually at the working site, so that the target refractory becomes softened there where it is contacted by molten or semi-molten refractory material which is either projected as such or results from combustion of the fuel. As a consequence of this, molten or semi-molten material impacting against the target surface adheres strongly to that surface and a dense coherent refractory weld mass is built up. It will be seen therefore that such a process is entirely unsuitable for the formation of a porous lining or repair.
There are other hot repair processes which are known and which have been used commercially. For example flame spraying processes are known in which a stream of refractory particles is projected from a burner nozzle in a combustible carrier gas, such as coal gas, which is mixed with oxygen at the burner outlet to form a flame which heats the refractory particles as they travel to the target surface. However such processes do not heat the refractory particles strongly enough for a satisfactory bond to be formed between the particles inter se or between the particles and the target surface. As a result, the refractory deposit formed has rather low resistance to abrasion.
Other processes for the repair of hot refractory structures which have been proposed include wet-gunniting and the plastering on of grog in a binder material. Again, such processes result in the formation of a repair mass which is only weakly bonded to the pre-existing structure, and such deposits can accordingly flake off rather easily.
Industry is accordingly faced with the problem of forming or repairing a porous thermally insulating refractory lining or wall while the lining or wall is hot, and in such manner as to preserve or afford good thermally insulating qualities.